Commonly, in the oil industry, it is necessary to treat wells with materials such as stimulation fluids. In certain stimulation processes, it is necessary for the fluids to be under enough pressure that they penetrate the rock surrounding the well, and that the fluids be dispersed at specific locations desired by the operator along the length of the well. This becomes especially necessary with horizontal wells which can have very long uncased portions requiring stimulation.
Traditionally, in the oil industry, two methods have been used to distribute treatment or stimulation materials into an uncased well. One method is the "bullhead" treatment method in which a jointed tubing is run to the bottom of the vertical or cased section of the well, and stimulation materials are pumped through the tubing, out of its bottom end, into the uncased portion of the well. This method has been successful in creating high enough pressures such that rock penetration, or "matrix acidizing" can be achieved, or the rock may be parted in a fracture treatment. However, studies have shown that this method offers very poor distribution of stimulation materials throughout the well, and does not allow the operator to determine where in the formation the stimulation materials are distributed.
Another method that has been used is the coiled tubing method, wherein a thin coiled tubing is run down the length of the well as stimulation materials are pumped through the coiled tubing. This method offers very good distribution of stimulation materials, however the coiled tubing method does not provide enough pressure such that the stimulation materials are able to adequately penetrate the matrix of, or fracture, the rock surrounding the well.
Another well stimulation system popular in the early 1980's was the "limited entry" system. This method was used exclusively with vertical, cased wells and involved perforating the casing at specific locations in the well. After a vertical well was completed with casing cemented in place, the casing was perforated in specific locations along its length and stimulation materials were pumped into the well. Thus, an operator could stimulate specific perforated portions of the vertical cased well. This method, however, did not allow any variance of the locations to be treated at a later time. The casing perforations were permanently in place, and if further locations were desired to be treated, the original perforations could not be removed without losing the pathway to the original treatment. The stimulated areas could only be added, and not varied. Thus, repeated stimulation of the same cased, vertical well in different locations resulted in a reduction of the desired distribution.
In uncased wells, the limited entry method is not available, especially when the uncased portion of the well is deviated. According to J. A. Short, author of Introduction to Directional and Horizontal Drilling, p. 215 (1993), fracturing and acidizing stimulation techniques are uncommon in horizontal open hole completions because "it is difficult to determine the amount and location where treating fluid enters the formation." Thus, for the foregoing reasons, there is a need for an improved stimulation method for use with deviated, uncased wells, providing the operator with a sufficient ability to distribute stimulation materials at particular locations in the formation while also providing adequate pressure such that stimulation materials can penetrate the matrix of or fracture, the rock if necessary.